1. Field of the Invention
The invention generally relates to transmitters. More particularly, the invention relates to transmitters that operate completely in the current domain to provide more “headroom” even in low voltage, high signal bandwidth scenarios.
2. Related Art
FIG. 1 shows a transmitter including, in series, a digital-to-analog converter (DAC) 102, a low-pass reconstruction filter (LPF) 104, an up-conversion mixer 106, a power amplifier (PA) 108, and an antenna 110. Conventionally, operations within the transmitter elements have been in the “voltage domain.” That is, signal voltages (as distinguished from currents) have been linearly related to the quantity that the signals represent. In this disclosure, such transmitters are termed “voltage mode transmitters.”
The low power supply magnitude required by deep submicron CMOS technology creates a “headroom” issue: signals cannot traverse a wide range without becoming non-linear with respect to the quantity that the signals are supposed to represent (usually, the baseband signal input to the transmitter). Lacking headroom not only means that linearity in general is limited, but also that the 1 dB compression point is reduced.
Generally, the current-voltage (I-V) transfer characteristics of CMOS (complementary metal oxide semiconductor) transistors is a quadratic relationship, with current being proportional to the square of voltage. Especially when the bandwidth of the baseband signal is high (in the hundreds of megahertz to a few gigahertz), a current steering DAC is employed. In this scenario, the baseband input signal is in the form of a current that must be converted eventually, possibly by modulation to a voltage (proportional to the square root of current) to drive an antenna or other load that is in the form of a linear or nearly linear resistance. Unfortunately, this current-to-voltage conversion requires not only special linearization techniques in the up-conversion mixer, but also demands extra biasing circuitry, including large capacitors, for DC level shifting.
Specifically, in conventional voltage mode operation, the input needs to be AC coupled to the next stage. As a result, not only must an AC coupling capacitor be inserted along the input signal path, but also a biasing circuit may be needed to properly bias the node on the downstream side of the AC coupling capacitor, and a degeneration network may have to be connected to the load to increase linearity.
Accordingly, conventional voltage mode transmitters are inherently complicated in design, and are especially unsuited for low voltage, high bandwidth applications that are of growing importance as semiconductor device sizes are reduced. Thus, there is a need in the art to provide a transmitter that is simple in design, yet provides signals that accurately represent quantities over a wide linear range and with expanded headroom.